.chat-msg {
    position: relative;
    width: 300px;
    min-height: 80px;
    padding: 10px 8px;
    border: 1px solid #ccc;
    border-radius: 4px;
    /* 借助 filter 添加阴影效果  filter(滤镜) 属性 */
    filter: drop-shadow(0 0 2px #999);
    background-color: #fff;
}

.chat-msg:before {
    content: '';
    position: absolute;
    top: 34px;
    left: -10px;
    border-top: 6px solid transparent;
    border-bottom: 6px solid transparent;
    border-right: 10px solid #ccc;
}

.chat-msg:after {
    content: '';
    position: absolute;
    top: 34px;
    left: -8px;
    border-top: 6px solid transparent;
    border-bottom: 6px solid transparent;
    border-right: 10px solid #fff;
}